Crosstalk reduction



Nov. 17, 1942. J. T. 1.. BROWN CROSSTALK REDUCTION Filed Feb. 24, 1942 2Sheets-Sheet l Kl. M)

T0 CATHODE HEATER OF VT lNVENTOP JZ'LBROWN MiW ATTORNEY Patented Nov.17, 1942 UNITED sTTs PATENT OFFICE onoss'rrirrr REDUCTION John T. L.Brown, Short Hills, N. J., assignor to Bell TelephoneLaboratories,Incorporated, New York, N. 1 a corporation of New YorkApplication February 24, 194-2, Serial No. 432,145

' 5 Claims.

The invention relates to a telephone transmission system andparticularly to a circuit for discriminating between transmitted wavesof different amplitudes in such a system.

It is known that crosstalk or other noise interference ismostobjectionable in telephone systems during pauses in telephoneconversation for during talking intervals it is to a large extent maskedby the transmitted voice signals. The maximum allowable amount ofcrosstalk is therefore determined by the amount permissible during thepauses in conversation.

An object of the invention is to rendera telephone circuit capable ofdiscriminating between transmitted waves of difierent amplitudes, forexample, between crosstalk or other noise interference and speechsignals.

. Another object is to reduce crosstalk or other noise interference in atelephone transmission circuit, without undue distortion of the speechsignals transmitted thereover.

. These objects-are attained in accordance with the invention by theusein the telephone circuit of a limited volume range expandercomprising a variable loss network in the speech transmission circuitproviding a normal loss sufficient to reduce the maximum amount ofcrosstalk to a negligible amount during non-speech intervals, and avoice-operated control circuit including a single tuberectified-reaction type detector'for reducing the loss value of thenetwork to a low value during talking intervals.

A feature of the invention is the use of a special connection betweenthe speechtransmis sion circuit and the detector input of such acircuit, providing close-coupling and thus high detector sensitivity forthe lowest amplitude speech iii inputs, and a less efficient couplingfor higher amplitude speech inputs to prevent introduction of loss anddistortion in the speech circuit.

The various objects and features of the invention will be betterunderstood from the following complete description when read inconjunction with the accompanying drawings in which:

Fig. 1 shows schematically a circuit embodying the invention; and I p VFigs. 2 to 4 show curves illustrating the operation of the circuit ofthe invention.

Fig. 1 shows a portion of a speech transmission circuit subject tocrosstalk or other noise interference, having input conductors I, 2 andoutput conductors 3,4. The crosstalk reducer of the invention comprisesa vario-losser VL con nected in that portion of the speech transmissioncircuit by input transformer TI and out-- 55 attenuation pad,the seriesarm of which includes in series between the two halves of the secondarywinding of transformer Tl, the equivalent op-' positely poledcopper-oxide varistors VI and V2, respectively, shunted by the equalresistors RI and R2 in series, and by the equal resistors R3 and R4 inseries, and the shunt arm of which comprises the equivalent oppositelypoled copper-oxide varistors V3 and V4 in series with the equalresistors R5 and R6, connected across the speech transmission circuitbetween the transformers TI and T2, the mid-point of this shunt armbetween the varistors V3 and V I-being connected to ground. i V Thecontrol circuit CL for the vario-lossenVL comprises a-single tubedetector of the rectifiedreaction type. It includes an input transformerT3 having two equal primary windings 5 and 6 respectively connectedacross the intermediate voltage tap l on the upper half of the secondarywinding of transformer TI and a point between the resistors RI aind R2shunting the series varistor Vl, and across an intermediate voltage tap8 on the lower half of the secondary winding of transformer TI and apoint between the resistors R3 and R4 shunting the seriesvaristor V2,and a secondary winding 9 in the control grid-cathode circuit of thepentode detector vacuum tube VT.

The tube VT may be a Western Electric No.

7727 tube which is particularly adapted for the required use as it hasexceptionally hightransconductance and its maximum plate plus screencurrent with zero bias and 130-volt plate battery is of the proper orderof magnitude for operation of the vario-losser. Instead of using theconventional pentode arrangement in this tube, the screen grid isconnected directly to the plate. The control grid of tube VT isconnected through the resistance R1, the secondary winding 9 of inputtransformer T3, and the parallel resistance-condenser combination 7 R8,C I in I se-. ries to a variable tap onthe potentiometer R9 (for.initially adjusting the detector sensitivity) shunting the 6-voltbatteryBl supplying heater current to the heater type cathode of thetube VT} The plate-cathode. circuit of the tube VT includes in seriesthe primary winding of feed back transformer T4, the righthand contactsof key KI and the 130-volt plate battery B2. I

To make the detector operate in'accordanc e with the rectified-reaction.principle, the plate cathode circuit of tube VT is coupled to thecontrol grid-cathode circuit through a feedback circuit comprisingfeedback transformer T4, rectifier bridge D and a resistance-condensercombination RIO, R8 and CI for controlling the time constant (attack andhangover characteristics). This feedback circuit operates to rectify theal ternating current output of tube VT and feed it back to the inputwith such polarity as to make the control grid of that tube morepositive, thus increasing the detector sensitivity after it has operatedin response to applied speech signals.

, With no speech input to the input conductors I, 2 of the speechtransmission circuit, the amount of biasing direct current flowingthrough the 2 series varistors VI and V2 and the shunt varistors V3 andV4 of suitable value is made such that these varistors in combinationwith the associated resistances Rl to R5 of proper values, form abalanced IO-decibel L attenuation pad with an output impedance matchingthat of the outgoing speech transmission circuit. The circuit impedanceat the vario-losser VL is normally 1500 ohms, being stepped up from theGOO-ohm impedance of the incoming speech transmission circuit by inputtransformer TI. The initial direct current bias on the shunt varistorsV3 and V4 is obtained by a connection from the mid-point of the primarywinding of transformer T2 through series resistor RH to a tap on thepotentiometer Rl2 shunting the grounded cathode heater battery B l whichis adjusted to provide about 1 milliampere total biasing current (.5milliampere in each varistor), making the alternating current impedanceof the varistors V3 and V4 about half of the total shunt impedance. Theinitial direct current bias on the series varistors VI and V2 is derivedfrom the detector by a conductive connection from its plate-cathodecircuit to a point between varistors VI and V2 and from its cathodeheater circuit through potentiometer RH and resistor RH to the mid-pointof the primary winding of transformer T2, the total initial detectorcurrent being adjusted to about .3 milliampere.

The major portion of this initial current flows '1 through resistancesRI, R2, R3 and R4, which shunt the series varistors, so that thesevaristors reduce theseries alternating current impedance or loss in thespeech transmission circuit from 2,000 ohms to about 1,600 ohms. forinputs to the speech transmission circuit below the operating point ofthe detector is therefore determined primarily by the fixed resistancesRl to R6, rather than by precise adjustment of initial detector andshunt bias current.

When the detector operates in response to the application of speechinput to input transformer T3, the current from it increases to amaximum value of about 24 milliamperes. This reduces the alternatingcurrent impedance of the series varistors VI and V2 to a negligiblevalue. The increased detector current, flowing through resistance Rl I,also reverses the direct current potential on the shunt varistors V3 andV4, causing their alternating current impedance to be increased to avery high value. As a result, the circuit loss in the speechtransmission circuit for full operation of the detector is reducedpractically to that corresponding to the loss in the transformers TI andT2, say about .8 decibel.

In order to obtain high detector sensitivity with respect to the speechinput, a fairly efiicient coupling between the detector and speechcircuit is required at the lower speech inputs. At higher inputs, closecoupling is undesirable because of The circuit loss :1.

loss and distortion introduced in the speech circuit when the detectorgrid draws current. Because of the high impedance ratio required in thedetector input transformer T3 and design limitations on the tube VT, itis not feasible to avoid trouble from this source by a large resistancein series with the detector grid. The maximum value for the seriesresistance R! should be about 250,000 ohms. The circuit has thereforebeen arranged so that the coupling between the speech channel and thedetector input is reduced when the detector operates. The input to thedetector through transformer T3 is derived effectively from twoalternating current voltage sources in series. One of these isapproximately half of the alternating current voltage across the seriesvaristors VI and V2 obtained from the taps between the resistors RI andR2 and between R3 and R4, and has a value which ranges from about 12decibels below the open circuit input voltage of the speech circuit atlow inputs to a negligible value at high inputs. The second voltage isobtained from the taps 1 and 8 on the secondary windings of inputtransformer TI and has a maximum value at high inputs corresponding toabout 32 decibels below the open circuit input voltage of the speechcircuit. The reduction in coupling on operation of the detector istherefore about 20 decibels. The coupling reduction is more thancompensated for in the tube VT by increase in transconductance withincrease in plate current. There is no discontinuity, therefore, in theincrease of plate current as the detector input is increased.

The direct current connection to the variolosser VL is in the cathodeleg of the detector tube-a feature necessitated by the use of currentfrom both the cathode and plate batteries in the vario-losser VL. Thelosser current from the detector is therefore the sum of the screen andplate currents of tube VT. The ratio between these two currents ispractically constant in the operating range. The capacity C2 is providedprimarily to reduce crosstalk between the detector circuit and thespeech circuit. It also has some effect on the attack and hangovercharacteristics of the detector circuit, primarily controlled by theresistance-condenser combination R1, R8, CI in the feedback circuit ofthe detector tube.

Key-Kl is provided for effectively switching the vario-losser in and outby substituting resistance R13 in the 130-volt circuit in place of thedetector tube VT when the key is operated to its lefthand contact. Thiscontrol can be either local or remote, the latter to be used in the caseof multiple link connections to avoid the effects of several crosstalkreducers operating in tandem.

The curves of Figs. 2 and 3 show the operating characteristics forvarious levels of 1,000 cycles r per second tone input to the crosstalkreducer of Fig. 1, curves for loss, output impedance, detector currentand current through the shunt varistors being included.

Fig. 4 shows curves of loss in the speech transmission circuit plottedagainst frequency provided by the crosstalk reducer of Fig. l for threedifferent values of detector current. The detector current selectedcorresponds to full detector operation (23.5 milliamperes), a valuecorresponding to operation on minimum speech volumes (17.6 milliamperes)and a value corresponding to practically no operation of the detector(.48 milliampere).

Referring to the curves of Figs. 2 and 3, it will be noted that most ofthe change in loss occurs between 40 dbm. (decibels referred to 1milliwatt) and 30 dbm., the loss at 35 dbm. being within about 2decibels of the minimum loss. The minimum loss, .8 decibel, occursbetween 5 and 20 dbm. As indicated, the loss again increases for inputsabove 5 dbm., because of the shunting efiect resulting from overloadingin the detector control grid circuit. About 8 decibels of the loss isremoved with a detector current of about 3 milliamperes. At a detectorcurrent of about 10 milliamperes, the current through the shuntvaristors reverses. The efiect of detector current increase between 10and milliamperes is very small, so that commercial variations in tubesand other elements determining the maximum current in the lesser shouldnot be important.

Tests with the circuit of Fig. 1 indicate that crosstalk may be reducedby about 7 decibels during continuous speech transmission and 10decibels during the longer pauses, with negligible effect on speechvolume or quality.

Various modifications of the circuit which has been illustrated anddescribed which are within the spirit and scope of the invention willoccur to persons skilled in the art.

For example, the coupling between the control grid and plate of thedetector tube VT may be made with a condenser in place of thetransformer T4 shown. The initial bias current in the shunt varistorscan be provided by fixed resistances instead of by an adjustablepotentiometer as shown. The crosstalk reducer circuit described wasdesigned for insertion in a cable carrier telephone system at the outputof a demodulator-amplifier, where the lever is about +4 decibels. Someeconomy may be effected by combining the output transformer of thedemodulator-amplifier and the speech input coil Tl of the crosstalkreducer in such a combination in a single unit. A switching loss ofabout 15 decibels might be desirable for some applications. It appearsthat this can be readily obtained by decreasing the values of theresistors R5 and'Rfi in the shunt arm and adding a second pair ofresistance-shunted series arm varistors, forming an H attenuation pad.

What is claimed is:

1. In combination with a transmission circuit transmitting speech signalwaves and subject to interfering noise waves of low amplitudes, such ascrosstalk, a variable loss network includ ng impedance elements havingnon-linear voltagecurrent characteristics, connected in said circuit, arectified-reaction type detector employing a single space dischargetube, having an input circuit supplied with a portion of said wavesapplied to said network, and an output circuit connected to saidnon-linear impedance elements in such manner that the detector outputcurrent controls the alternating current impedance of said elements andthus the loss value of said network in said transmission circuit, saiddetector and said non-linear impedance elements being normally biased sothat, in the absence of applied speech waves, said network inserts afixed loss in said speech transmitting circuit sufiicient to reduce thetransmission of said noise waves of said low amplitude to a negligibleamount, the output current of said detector being increased sufilcientlyin response to the application of speech waves to its input to reducethe attenuation of said loss network to a low value.

2. The combination of claim 1 in which the connection between saidspeech transmission circuit and the input circuit of said detector issuch as to provide an efficient coupling therebetween for low amplitudespeech inputs and a less ellicient coupling therebetween for higheramplitude speech inputs.

3. The combination of claim 1 in which said variable loss networkcomprises a balanced L type attenuation pad including non-linearimpedance elements shunted by linear impedance elements in the seriesarm and non-linear impedance elements in series with linear impedanceelements in the shunt arm, the output current of said detector passingthrough said linear impedance elements to control the voltage applied toand thus the alternating current impedance of the associated non-linearimpedance elements in accordance with the amount thereof.

4. In combination with a circuit transmitting speech signal waves andsubject to interfering noise waves of low amplitudes, a variable lossnetwork including impedance elements having a non-linear voltage currentcharacteristic, connected in said circuit, a detector having an inputcircuit supplied with a portion of said waves applied to said networkand an output circuit connected to said network in such manner that thealternating current impedance of said non-linear impedance elements, andthus the loss value of said network depends on the amount of detectoroutput current, said non-linear impedance elements and said detectorbeing normally biased so that, in the absence of speech wavetransmission,

said network provides a loss in said circuit of sufficient value toreduce the transmission of waves of said low amplitudes to a negligibleamount, and the increase in detector output current when speech wavesare applied to the input thereof causing the loss value of said networkto be reduced to a low value and means for providing an eificientcoupling between said speech transmission circuit and the input circuitof said detector for the lowest amplitude speech wave inputs and a lessefiicient coupling therebetween for higher amplitude speech wave inputs.

5. The combination of claim 4 in which said loss network includes aninput transformer having two secondary windings and oppositely poledcopper-oxide varistors each shunted by a linear resistance element,connected in series between said secondary windings, and said means forcoupling said speech transmission circuit and the input circuit of saiddetector comprises a transformer therebetween having two separateprimary windings, the terminals of one of which are connected across amid-tap on the resistance element shunting one of said series varistorsand an intermediate voltage tap on one of the secondary windings on saidinput transformer, and the terminals of the other of which are connectedacross a mid-point of the resistance element shunting the other of saidseries varistors and an intermediate voltage tap on the other secondarywinding of said input transformer.

JOHN T. L. BROWN.

